Hybrid assemblies and printed circuits utilize a ceramic substrate or a glass filled epoxy substrate, respectively, to which a conductive metal layer, such as a copper layer, has been applied so that metallized conductor patterns can be formed. The thickness of the conductive layer can be built up thereafter by conventional plating with the same metal. Alternatively, after coating the substrate with a metal such as copper, conventional plating with other metals, such as nickel or gold, can follow to build up the thickness of the conductive layer.
The state-of-the-art to date consists of either screen printing usually precious metal inks on the ceramic substrate or depositing very thin layers of vacuum deposited metallization to form the conductive circuit patterns. Both techniques are costly. However, a very large market has developed demanding both less expensive methods and more efficient circuitry.
A thin film circuit on ceramic typically consists of a thin film of metal deposited on a ceramic substrate by one of the vacuum deposition techniques. In these techniques, generally a chromium or molybdenum film, having a thickness of about 0.02 micrometers (.mu.m), acts as a bonding agent for copper or gold layers. Photolithography is used to produce high resolution patterns by etching away the excess thin metal film. Such conductive patterns may be electroplated up to, typically, 7 .mu.m thick. However, due to their high costs, thin film circuits have been limited to specialized applications such as high frequency and military applications where a high pattern resolution is vital.
Another method of producing printed circuits is known as the thick film method. A thick film printed circuit comprises a conductor pattern consisting of a metal and glass frit and/or metal oxide particles fired on a ceramic substrate. Typically, the film has a thickness of about 15 .mu.m. Thick film circuits have been widely used and are produced by screen printing on a circuit pattern with a paste containing the conductive metal powder and a glass frit and/or metal oxide particles in an organic carrier. After printing, the ceramic parts are fired in a furnace to burn off the carrier, sinter the conductive metal particles and fuse the glass. These conductors are firmly bonded to the ceramic by the glass and thus components may be attached to the conductors by soldering, wire bonding and the like.
A disadvantage in using thick film printed circuits is that the conductors have only a 30 to 60 percent of the conductivity of the pure metal. In general, there is a need for the higher conductivity attainable by pure metal in order to provide the necessary conductive paths for higher density circuits or greater power carrying capabilities.
Another disadvantage with regard to thick film printed circuits is that the minimum conductor width and minimum space between conductors which can be obtained by screen printing and firing under these procedures is generally 125 and 200 .mu.m, respectively, even under special high quality procedures. Under normal production conditions, these minima are 200 and 250 .mu.m, respectively, which restrict denser circuit design considerations. In addition, thru-hole metallization is difficult due to the inconsistency of the paste process to conform readily to the internal characteristics of the holes in the ceramic substrate.
For ceramic surfaces requiring higher interconnection density, multi-layer techniques are often used. In order to produce thick film multi-layer circuits, a first layer of metal powder and glass frit is printed on a ceramic substrate and fired, typically at 850.degree. C. Subsequently, an insulating dielectric layer is screened over the conductor pattern, leaving exposed only the points at which contact is to be made to the next layer of metallization. This dielectric pattern is also fired at approximately 850.degree. C. (about 1562.degree. F.). This procedure is repeated to produce second and subsequent conductive or dielectric layers until the desired number of layers has been produced to provide the circuit density required.
Attempts also have been made in the past to directly bond pure metal conductors to ceramic substrates including alumina in order to achieve higher conductivity for ceramic-based circuit patterns. Illustrative of such attempts is U.S. Pat. No. 3,994,430 to Cusano et al. which describes a process which involves the bonding of copper sheets to alumina by heating the copper in air to form an oxide layer on its surface. The prepared copper sheet is then bonded to the alumina at a temperature between 1065.degree. C. and 1075.degree. C. in a nitrogen furnace. However, in order to produce a well adhered copper foil with minimum blisters, the process parameters have to be controlled very closely, which is hard to do under commercial operating conditions.
Another process for the direct bonding of metal sheets to non-metallic substrates is described in U.S. Pat. No. 3,766,634 to Babcock according to which a metal foil is juxtaposed on a ceramic substrate and heat-bonded thereto allegedly utilizing a copper-copper oxide eutectic that is formed during the bonding process. However, this method suffers from many of the same problems as the aforesaid Cusano et al. process, including the inability to provide through hole coverage, the entrapment of gases which cause blistering between the metal layer and the substrate, as well the inherent limitations on the ability to provide narrow conductive metal traces that are sufficiently adherent to the substrate for efficient handling and processing.
Although the above described metallization systems have been commercially used, the need for direct, simple metallization of ceramics for producing pure metal conductors, such as copper, without deficiencies has prompted a continuing series of patents and proposed processes that utilize electroless deposition of an initial conductive metal layer onto the ceramic substrate. For example, U.S. Pat. No. 3,296,012 to Stalnecker, Jr. discloses a method of producing a microporous surface for electrolessly plating alumina.
U.S. Pat. No. 3,690,921 to Elmore involves the use of molten sodium hydroxide to etch a ceramic surface. In this process, subsequently, sodium hydroxide is rinsed from the ceramic surface with water, the ceramic surface neutralized with dilute sulfuric acid and rinsed again before sensitizing with a stannous chloride solution, rinsing and seeding with a palldium chloride solution, to catalyze the surface for electroless metal plating.
Although the process of Elmore provides good surface coverage it reached only limited acceptance for commercial production because of low adhesion. Moreover, the alkaline surface treatment undermines the substrate surface structure. Although the metal deposit usually covered 90 percent of the surface area or even more, see for example U.S. Pat. No. 4,574,094 to DeLuca et al at column 3, this was unacceptable since any imperfection in the foreign metal film may cause defective circuit conductors and a failure of such imperfection occurs in a fine line conductor pattern.
U.S. Pat. No. 4,428,986 to Schachameyer discloses a method for direct autocatalytic plating of a metal film on a beryllia substrate. The method comprises roughening the surface of the beryllia substrate by immersing it in a 50 percent sodium hydroxide solution, rinsing with water, etching the beryllia substrate with fluoroboric acid, rinsing the water, immersing it in a solution of 5 g/l stannous chloride and 3N hydrochloric acid, rinsing with water, treating the beryllia surface with a 0.1 g/l palladium chloride solution, rinsing with water and then electrolessly plating nickel on the beryllia. However, the method of Schachameyer has shortcomings as discussed hereinbelow.
When depositing copper metal on a ceramic substrate, the strength of the copper-ceramic bond is very important. This is because if the adhesion between the two substances is weak, the copper may peel off of the ceramic upon normal thermal cycling, thus rendering the component worthless. In the etching step of Schachameyer, silica and magnesium are removed from the grain boundaries of the beryllia thereby weakening the beryllia surface and as a result, the method of this patent was able to achieve only 1.7 MPascal bond strength. This bond strength is too low for practical use required for thick film type circuits.
Another method of applying a layer of a conductive metal such as copper onto a ceramic substrate is described in U.S. Pat. No. 4,574,094 to De Luca et al. According to this process the ceramic surface to which a conductive metal layer is to be adhered is first etched with an alkali metal preparation, then prepared for electroless deposition of the conductive metal layer, and followed by the electroless deposition of such metal layer. Nevertheless, the metal-to-ceramic bond strength that can be achieved by this technique is not adequate for the produced metal ceramic composite to withstand repeated firings at thick film firing temperatures of about 850.degree. C. (about 1562.degree. F.).
The prior art processes heretofore available have been unreliable as they resulted in incomplete surface coverage with, or unacceptably low bond strength of, a formed metal deposit, or both, and are therefore unsatisfactory for commercial product purposes. However, it is desirable for economic and increased circuit design capability reasons to have metallization adhesion to the substrate that will withstand subsequent high temperature thick film processing firings. Attempts heretofore to reproducibly make circuit patterns by direct autocatalytic deposition have only experienced limited success due to inadequate adhesion of the metal layer. Inadequate adhesion results in an inability of the layer to withstand subsequent exposure to thick film processing temperatures. However, the present invention satisfies the foregoing desires by providing an electrolessly deposited, direct bonded conductor having excellent adhesion that can withstand elevated thick film temperature firings and a process for producing the same. Concurrently, thru-hole conductor paths can be provided, if desired, without secondary operations to the electrodeposition process.